Semiconductor cutting technology is evolving from diamond tool cutting to ordinary laser cutting. In laser cutting, the wavelength is generally 355 nm or 266 nm, sufficient to scribe sapphire substrates and various film layers, such as GaN layers, Bragg reflective layers and metal layers. In cutting, a surface cutting channel can be formed by scribing the epitaxial wafer surface with laser. Or, an invisible cutting channel can be formed by focusing a laser inside the substrate to separate a single chip.
In the first case, impurities from laser burning would adhere to the side wall of the cutting channel, which blocks light emission and affects external quantum efficiency (EQE) of the semiconductor element. Therefore, to avoid effects on luminance, after a laser cutting channel is formed, the side wall of the cutting channel is cleaned with immersion and corrosion of chemical solution to remove the laser burning impurities adhered thereto.
Physical vapor deposition (PVD) is an easy-to-operate process that consumes little raw material and causes little environmental pollution. Moreover, the film formed via PVD is dense and even, and is tightly bonded to the base. With these advantages, PVD is increasingly applied in fabrication of semiconductor elements, in particular, to fabrication of bottom layers of epitaxial wafer. For example, by depositing an AlN layer as the buffer layer, defects from lattice mismatch and thermal mismatch between the substrate and the epitaxial layer can be minimized, and the stress caused therefrom can be eased, thus improving quality of the semiconductor element.
However, the multi-lattice and anisotropic AN layer formed via PVD is vulnerable to corrosion by chemical solution. Therefore, if an epitaxial wafer comprises a buffer layer formed via PVD, when the epitaxial wafer surface is cut by a laser and then immersed in chemical solution, the contact surface between the buffer layer and the substrate is prone to over-corrosion, leading to abnormal electricity of chip and low production yield.